Most commercial fertilizers, such as urea, dissolve quickly in water. When applied to lawns or agricultural crops, most of the fertilizer dissolves rapidly in the moisture in the soil. Often, such fertilizers dissolve at a rate that exceeds the rate at which it can be used by plants. This can cause a number of significant problems. First, the excess fertilizer can leach into the groundwater, where it can cause potentially serious environmental damage. In addition, excessive concentration of fertilizer in the vicinity of a plant may result in “burning” of the plant roots.
Because of these problems, it is often necessary to apply these fertilizers in several light doses throughout the growing season, rather than in a single heavy application. However, the need for repeat applications increases labor costs. Worse, repeat applications may cause physical damage to the growing plants because the fertilizer spreading equipment must pass over the plants several times.
In order to avoid the need for repeat applications, various slow or controlled release fertilizers have been developed. For examples, see U.S. Pat. Nos. 4,042,366, 5,147,443, 5,435,821, and 6,231,633. These fertilizers can be applied at the beginning of the growing season and provide an initial high release of fertilizer followed by a slower, gradual release throughout the growing season. Slow or controlled release fertilizers are widely used on home lawns, public lawns, golf courses, home gardens, plant nurseries, and on horticultural crops.
Polymer coated fertilizers are the most technically advanced controlled release fertilizers. Different methods of manufacturing are utilized to produce polymer coated fertilizers. For examples, see U.S. Pat. Nos. 4,019,890, 4,369,055, 5,176,734, 5,211,985, 5,374,292, 5,858,094, and 6,338,746. Polymer coated controlled release fertilizers are typically produced by coating water insoluble semi-permeable polymer layer onto the fertilizer granules. Over time the fertilizer is released into the soil through diffusion from this semi-permeable coating. One of the disadvantages in existing methods is that uniform polymer coating thickness is difficult to achieve. Another disadvantage is that it is hard to control the thickness of the polymer that is applied the fertilizer. Yet another disadvantage is that most of these manufacturing processes are batch type processes which are expensive. Yet another disadvantage with existing processes is that they require multiple applications of polymer to increase the thickness of the polymer coating around the fertilizer.